Electropickling process



United States Pater G ELECTROPICKLING PROCESS Arthur L. Hart, Cary, andGilbert G. Kamm, Barrington, 11]., assignors to American Can Company,New York, N. Y., a corporation of New Jersey No Drawing. ApplicationDecember 31, 1957 Serial No. 706,291

4 Claims. (Cl. 204-143) phosphate, chromate, etc., and other undesirablematerials from a black iron surface using electrical energy to speed upthe heating and pickling operations of the process.

The instant invention will be described in relation to treating themargins of black iron can body blanks. However, it is to be understoodthat the invention is limited only by the scope of the claims sincemodifications will be readily apparent to those skilled in the artwhereby other shapes may be treated Without departing from the spirit ofthe invention.

Before the longitudinal margins of a black iron, i. e. untinned, lowcarbon steel, can be bonded such as by soldering or welding to form atubular can body, it is necessary that any coating such as ironoxidation products, grease, dirt and other foreign matter be removedfrom the surface of these margins. Since commercial practice demandsthat can bodies be made at a speed of 400 per minute or higher, verylittle time is available to accomplish the necessary cleaning operation.

It is, therefore, an object of the present invention to provide aprocess for the removal of oxidation products of iron, dirt, grease andother foreign matter from the surface of a ferrous metal object at highspeed.

It is also an object of the invention to provide a process of thecharacter described which is well suited to cleaning the longitudinalmargins of a black iron can body blank at commercially acceptablespeeds.

Numerous other objects and advantages of the invention will be apparentas it is better understood from the following description which is of apreferred embodiment thereof.

The present invention achieves the above objects by first inductivelyheating the surface to be cleaned to a temperature within a criticalrange, then immersing the heated, surface in an aqueous solution of astrong acid fora brief time interval, the concentration of the acid andthe temperature of the solution being within a critical range, andpassing an alternating current having a density within a critical rangethrough the solution while the surface, which is one electrode in theelectrochemical circuit, is immersed therein. Thereafter, the cleanedsurface is washed free of the pickling solution and dried. As a resultof this procedure, involving merely a matter of seconds, subsequentoperations such as tinning, soldering or welding, may be readily andefficiently performed upon the treated surface.

The induction heating step is especially necessary and critical toobtain the desired results of the instant invention. After extensiveexperimentation, it was determined that to obtain rapid and efficientcleaning of a black plate surface in the electropickling operation, thesurface must be heated to a temperature of from 450 F. to 550 F. andpreferably 525 F. before immersion in the Patented Nov. 11, 1958 icepickling solution. It was found that the only means of raising thesurface to the necessary temperature in a commercially feasible timeinterval is by inductively heating the surface using an inductionfrequency of from 10 kilocycles to l megacycle per second, andpreferably from to 500 kilocycles. The lower induction frequencies arepreferred so that lower voltagesprevail at the coil surrounding the workthereby reducing the danger of arcing between this work coil and thework. With the induction heating step of the present invention, a timeinterval of only .25 to 1 second, and more particularly only .4 second,is required to raise the margins of a black iron can body blank to thenecessary temperature.

An additional advantage obtained by use of induction heating over otherforms of heating such as gas flames and infra red, is that the heat isconfined to the work being treated and does not heat adjacent machinery.The value of this advantage in preventing wasting of energy anddetriment to the adjacent machinery is ob VlOUS.

Induction coils for preforming induction heating are Well known in theart and, therefore, deemed unnecessary of description. These coils maytake various shapes depending upon the shape of the work being treated.United States Patent No. 2,647,981 discloses various shapes andpositioning of induction coils for the heating of a metal surface,including a flat surface- I The strong acid in the aqueous picklingsolution is hydrochloric acid. It is present in a weight concentration,at least initially, of about 15% to 20% and preferably about 18%. Thepickling bath may also contain other optional ingredients in smallamounts such as: a non-foaming wetting agent, for example, 0.1% of apolyethylene glycol alkyl and ether (Triton X400).

The pickling bath must be maintained at a temperature of about F. to 150F. and preferably about F. to preform the rapid and eflicient cleaningof the instant invention. The necessary heat may be im parted to thebath by means of gas burners, electric resistance heaters or the like.However, it has been found that after a relatively short interval ofinitial operation, the hot body blanks as they are immersed in thepickling bath supply sufficient heat to the bath to maintain it at thenecessary temperature thereby obviating the need for auxiliary heating.If desired, the initial warm-up period can be shortened by heating thebath such as by gas burners until the desired temperature is obtained.

In the electropickling process, the work constitutes one electrode andthe other electrode is formed from an inert, electrically conductivematerial, preferably carbon. The best arrangement for carrying out thisprocess is to have a pair of carbon electrodes connected in parallelspaced 2 to 3 inches apart with the work, i. e. the marginal edgeportion of the can body blank, perpendicular to a line between thecarbon electrodes and centered between them. The 2 inch minimum isnecessary to minimize resistance differences between the Work and itsopposed carbon electrodes caused by slight variations in the position ofthe work relative to its opposed carbon electrodes. Substantialdifferences in resistance between each side of the work in relation toits adjacent carbon electrode prevents even distribution of the current,thereby causing uneven cleaning of one side of the work relative to itsopposite side. Spacing of the carbon electrodes greater than 3 inchesapart, although operable, is commercially impracticable because ofequipment limitations and the greater power consumption occasionedthereby.

As is well known, the polarity of the work and carbon electrodesreverses cycylically due to the use of alternating current in theelectropickling operation. The chemical reactions at each of theelectrodes is well known in the art.. Suflice it to say that when thework is the anode,

metal e. g. iron if the work is composed of steel, is

dissolved from its submerged surface; and when the work is the cathode,hydrogen is liberated at its surface.

'These reactions on the surface of the work greatly facilitate thepickling or cleaning operation in that the dissolution of the base metaltends to loosen or undermine any surface film while the hydrogen gasformed performs, in effect, a scrubbing action whereby the .loosenedsurface film is removed.

. The current density used in the present invention is from 500 to 2000amperes per square foot, and preferably about 1500 amperes per squarefoot of work surface to I "2,860,092 V T V between induction coils,heating said surface to a temperature of about 500 F. to 550 F. in about.25 to 1 second by means of said coils, immersing the thus heatedsurface in an aqueous HCl solution for about from 2 to 4 seconds, saidsolution containing 15% to 20% by weight of HCl and being maintained ata temperature of from 110 F. to 150 F., making said surface oneelectrode in an electrochemical circuit, and passing an alternatingcurrent having a current density of from 500 to 2000 amperes per squarefoot through said solution whereby said surface is cleaned anddeoxidized by an electropickling action.

2. The method set forth in claim 1 wherein said surface is the marginaledge portion of a can body blank.

3. The method set forth in claim 1 wherein the induction frequency insaid induction heating coils is from 10 kilocycles to one megacycle.

4. A method of high speed cleaning and deoxidizing the marginal edgeportion of a ferrous can body blank, comprising passing said portionbetween induction heating coils having an induction frequency of from100 to 500 kilocycles, heating said portion to a temperature of an airknife, followed by washing of the surface by spraying or flooding withwater and thereafter dried such as by a squeegee or air knife. Thecleaned surface is then in condition for a desired subsequent operationsuch as tinning, soldering or welding.

It is thought that the invention and many of its attendant advantageswill be understood from the foregoing description, and it will beapparent that various changes may be made in the steps of the processdescribed and their order of accomplishment without departing from thespirit and scope of the invention or sacrificing all of its materialadvantages, the process hereinbefore described being merely a preferredembodiment thereof.

We claim:

1. A method of high speed cleaning and deoxidizing a ferrous metalsurface, comprising passing said surface about 525 F. in about 0.4second by means of said coils, immersing the thus heated portion in anaqueous HCl solution for about 3 seconds, said solution containing about18% by weight HCl and being maintained at a temperature of about F.,making said can body blank one electrode in an electrochemical circuit,and

passing an alternating current having a current density of about 1500amperes per square foot through said solution whereby said portion iscleaned and deoxidized by an electropickling action.

References Cited in the file of this patent UNITED STATES PATENTS

4. A METHOD OF HIGH SPEED CLEANING AND DEOXIDIZING THE MARGINAL EDGEPORTION OF A FERROUS CAN BODY BLANK, COMPRISING PASSING SAID PORTIONBETWEEN INDUCTION HEATING COILS HAVING AN INDUCTION FREQUENCY OF FROM100 TO 500 KILOCYCLES, HEATING SAID PORTION TO A TEMPERATURE OF ABOUT525*F. IN ABOUT 0.4 SECOND BY MEANS OF SAID COILS, IMMERSING THE THUSHEATED PORTION IN AN AQUEOUS HCI SOLUTION FOR ABOUT 3 SECONDS, SAIDSOLUTION CONTAINING ABOUT 18% BY WEIGHT HCI AND BEING MAINTAINED AT ATEMPERATURE OF ABOUT 120*F., MAKING SAID CAN BODY BLANK ONE ELECTRODE INAN ELECTROCHEMICAL CIRCUIT, AND PASSING AN ALTERNATING CURRENT HAVING ACURRENT DENSITY OF ABOUT 1500 AMPERES PER SQUARE FOOT THROUGH SAIDSOLUTION WHEREBY SAID PORTION IS CLEANED AND DEOXIDIZED BY ANELECTROPICKLING ACTION.